Photodisintegration (also called phototransmutation) is a physical process in which an extremely high energy gamma ray interacts with an atomic nucleus and causes it to enter an excited state, which immediately decays by emitting a subatomic particle. A single proton or neutron is effectively knocked out of the nucleus by the incoming gamma ray. This process is essentially the reverse of nuclear fusion, where lighter elements at high temperatures combine together forming heavier elements and releasing energy. Photodisintegration is endothermic (energy absorbing) for atomic nuclei lighter than iron and exothermic (energy releasing) for atomic nuclei heavier than iron. Photodisintegration is responsible for the nucleosynthesis of at least some heavy, proton rich elements via p-process which takes place in supernovae.

A photodisintegration reaction

was used by James Chadwick and Maurice Goldhaber to measure the proton-neutron mass difference. This experiment proves that a neutron is not a bound state of a proton and an electron, as had been proposed by Ernest Rutherford.